This invention relates to a circuit for driving a solenoid at a high speed using a choke coil and, more particularly, to a high speed solenoid driving circuit with a choke coil adapted to abruptly rising an electric current flowing through the coil of the solenoid by means of the constant current characteristic of the choke coil and of the current switching characteristic of a transistor.
A method of converting an electric signal into a mechanical movement via a solenoid is widely used heretofore due to the inexpensiveness and the simple mechanical structure. There are generally used in the utilization of the electromagnetically attracting force of the solenoid a method of mechanically operating by a switch, and a method of controlling an electric current flowing through the coil of the solenoid in accordance with the voltage of a signal source by utilizing the switching characteristic of a transistor, since the latter method can operate the solenoid without a manual switch, it is used for the case of operating a number of operations within a predetermined time. Inasmuch as this method however employs a solenoid coil having an inductance, an electric current flowing through the coil of the solenoid cannot abruptly change, and this method accordingly has such drawbacks that the rising characteristic of the current becomes worse particularly when an electromagnetic force is produced at the solenoid.
FIG. 1 shows a conventional solenoid driving circuit of a resistance driving type having a resistor R.sub.1. When a DC current (12 volts) is supplied from a battery B through a switch SW such as, for example, a transistor or the like to this driving circuit, even if a signal S rises as designated by a broken line in a current flow curve in FIG. 2, an electric current flowing through a solenoid coil SC will rise as designated by a curve i of solid line in a predetermined primary delay characteristic to approach a predetermined content value. That is, since the time constant t.sub.1 representing the rising speed of the current at this time is signified by L/R, the value of the resistor R.sub.1 should be increased so as to shorten the time constant t.sub.1, but if the resistance of the resistor R.sub.1 is thus increased, the value of the current becomes small and the magnetic force of the solenoid coil accordingly becomes small. Therefore, the movement of an actuator attracted by the solenoid becomes worse. Consequently, the rising speed of the current is thus limited. If the value of the inductance of the L is decreased so as to shorten the time constant t.sub.1, the value of the current also becomes small and the magnetic force of the solenoid coil accordingly becomes small. Resultantly, the movement of the actuator attracted by the solenoid also becomes worse. In order to heretofore improve such a delay in raising the current flowing through the solenoid, a voltage applied to the solenoid coil at the time of rising the current flowing through the solenoid coil is increased in a trend. Particularly when a battery voltage is utilized for a vehicle or the like, a power source for raising the voltage should be additionally provided, and results in the increase in the cost and the complexity of the circuit and further in the power loss in the circuit as its drawback.